The intermediate wavelength UV region of wavelengths from 280 to 320 nm of the UV light from the sun causes scarlet patches called sunburn on skin, and in severe cases these may be accompanied by bubbles like in burn injuries. Wavelengths in the long UV region from 320 to 400 nm darken the skin, and it is known that repeated exposure over long periods of time to either of these wavelength regions accelerates skin aging.
To prevent the adverse effects of UV radiation on skin, sunscreen cosmetics that include a UV radiation absorbing agent or UV radiation protection powder are used.
Microparticle zinc oxide powder has been employed as one such material that effectively protects against the long wavelength UV region and is highly transparent to the visible spectrum. Further, it is known that W/0-type emulsified compositions are good because of their antiperspirant activity.
To blend microparticle zinc oxide in a W/O type emulsified composition, it is necessary to treat the surface of the microparticle zinc oxide in order to ensure the stability of the formulation. When microparticle zinc oxide surface is not processed uniformly, however, the elution of zinc ions adversely affects the stability of the system, thus making it difficult to process the surface of microparticle zinc oxide and stably blend microparticle zinc oxide into W/O type emulsified compositions.
On the other hand, the technology of providing pigment surfaces with the ability to repel water or both water and oil in order to keep the cosmetic from coming off, and thereby render the pigment resistant to sebum, is known in the field of makeup cosmetics. For example, Patent Documents 1 through 4 disclose technologies related to surface processing with perfluoroalkyl phosphate ester compounds. Patent Document 5 mentions a cosmetic to which has been blended fine particles whose surface has been coated by an acrylsilicone graft copolymer so as to improve the cosmetic's ability to adhere to skin and its usability.
Patent Document 6 proposes a technology that further improves upon the above technologies, in which a cosmetic pigment powder is treated by a phosphate ester of the general formula (1) and an ester of the general formula (2), which are used in the present invention, to render its surface hydrophobic.
The hydrophobic pigment disclosed in Patent Document 6 can be easily dispersed within a cosmetic, and its stated aim is to have the effect of producing a moist sensation when used. Paragraph [0019] of the disclosure enumerates many examples of inorganic pigments, organic pigments, and resin powder pigments as various types of cosmetic pigments whose surface is to be coated, and in its embodiments Patent Document 6 specifically discloses cosmetics that include pigment particles of the pigments titanium oxide, sericite, mica, talc, yellow oxides of iron, red ocher, and black oxides of iron that have been rendered hydrophobic.
However, Patent Document 6 provides no description whatsoever regarding methods for treating the surface of microparticle zinc oxide powder, which in general is unlike pigment powder in that it has a high degree of transparency and is an agent that disperses long wavelength UV light, and further offers absolutely no teaching regarding the new aims, and the conspicuous effects thereof, of lowering the absorbed oil amount and lowering the apparent specific volume of the microparticle zinc oxide powder that is brought forth by the present invention.
On the other hand, sunscreen cosmetics such as sunblock are used at pools and beaches, and thus there is a need for such cosmetics to have the ability to repel water and oil and to have excellent long lasting coverage (water-repelling and oil-repelling characteristics) so that it is not necessary for the cosmetic to be applied repeatedly. For this reason, water-in-oil emulsified cosmetics that include hydrophobic particles have been used as sunscreen cosmetics. Examples of water-in-oil emulsified cosmetics that include hydrophobic powder can be found in Patent Documents 7 through 10. The technique generally used to increase the long lasting coverage is to blend oils or resin components that produce more a refreshing sensation than ordinary oils when used and that have a high water-repelling effect.
There has been the problem that these components persist after use and are difficult to wash off in the shower, for example.
Patent Document 1: JP 2724257 B
Patent Document 2: JP 2672913 B
Patent Document 3: JP H5-86984 B
Patent Document 4: JP H3-246210 A
Patent Document 5: JP H5-339125 A
Patent Document 6: JP 2001-302455 A
Patent Document 7: JP 2691654 B
Patent Document 8 JP H11-246330 A
Patent Document 9: JP H9-202714 A
Patent Document 10: JP H6-321735 A